There is a mite called a depluming mite, and I don't believe it is visible to the naked eye. This mite causes them so much irritation they pull their own feathers out. I would get the ivermectin pour-on and treat this poor gal just in case. Make sure she is getting plenty to eat (not getting driven off by the others) and hand feed her some high-protein treats as mentioned above. Watch carefully to see if any others are pecking at her.
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